Many applications require accurate sizing and counting of particles, powders, granular substances, aerosols, blood cells, or any material that has a component which is made up of discrete parts of well defined dimensions. Hereafter, such materials will be referred to as particulates or particulate matter.
One known apparatus for sizing and counting of particulates is manufactured by Coulter Electronics, Hialeah, Fla. A particle counter of this type operates with an electrolyte bath and includes a constant current source, a voltmeter, memory circuits, a mechanical stirrer, electrodes, and a tube having a small inlet hole therein for the passage of fluid. The tube is immersed in the electrodes and a vacuum pump is connected to the upper end of the tube to draw fluid. One of the electrolytes is positioned in the tube, and the other is positioned in the electrolyte bath, such that the inlet hole lies between the electrodes in the fluid flow path.
The substance to be sized and counted is first immersed in a conductive electrolyte, such as aqueous NaCl, in order to make a very dilute suspension. A constant current is supplied between the electrodes, and the mechanical stirrer maintains the particulate matter well dispersed. The vacuum source draws the suspension through the small inlet hole into the tube. As a particle in the suspension passes through the inlet hole, a voltage pulse is generated that is directly proportional to the volume of the particle. The number of pulses accumulated represents the particle count, and the magnitude of each pulse or count is measured. In order that the measurements have an absolute (as opposed to relative) meaning, the pulses are calibrated against materials whose sizes have been determined by other methods, such as microscopic analysis. Thus, only through this two step process can both the actual sizes of the particles, as well as the count, be determined.
While the foregoing apparatus and method may be used to measure with some degree of accuracy the particle size and count, there are four basic problems associated with its usage: (a) the calibration techniques used do not always assure accurate calculation of particle size, and therefore the size distribution of particles may be erroneous; (b) materials used to calibrate the process against size determinations by other methods are not always readily available, are expensive, and often not well suited for such purpose; (c) the volume-proportional response of the system is only claimed to be a valid assumption for particulate matter having a maximum linear dimension of from 2 percent to 40 percent of the maximum cross sectional linear dimension of the inlet hole; and (d) particulates having a linear dimension larger than the minimum cross sectional linear dimension of the inlet hole tend to plug the inlet hole, sometimes resulting in costly damage, and in any case rendering the analysis inaccurate.
In present analysis work, the first two problems set forth above were considered inherent, and unless alleviated through considerable time and expense were merely tolerated. The remaining two problems are usually dealt with by extensive sample preparation to separate the sample into ranges of particulates of suitable dimensions for given inlet hole sizes so that by selection of an appropriate inlet hole size for a given size range, accurate measurement results can be obtained without the danger of clogging the inlet.
Usually such sample preparation falls into one of two broad categories: fluid calssification or mechanical classification. Fluid classification would include such devices and processes as elutriators against gravity and fluid classifiers against the centrifugal field. Mechanical classification would include such processes as wet or dry sieving. Unfortunately, either general method may lead to unrepresentative sample splitting because of large surface areas present, for example of the sieve, onto which fine particulates adhere. Since the prepared sample is no longer characteristic of the particulate material from which the sample was taken, a characteristic measurement cannot be performed.